Sterilization is an act of destroying all forms of life on and in an object. A substance is sterile, from a microbiological point of view, when it is free of all living microorganisms. Sterility is achieved with respect to all viable bacteria and viruses when they are killed and with respect to spores when they are unable to reconvert to a living thing. Sterilization is used to prevent the transmission of diseases between and among patients and dental and medical personnel by destroying microbes that may cause them in humans and animals.
By far the most resistant of all forms of life, to both physical and chemical killing agents, are some of the bacterial endospore, which have relatively little water content. If they did not exist, sterilization of such materials as bacteriological media and equipment, hospital supplies, and medical, dental and surgical implements would be simpler.
Microorganisms can be killed either by physical agents, such as heat, or by chemicals. Regardless of the manner in which they are killed, the microorganisms generally die at a constant rate under specified environmental conditions. This death rate can be expressed exponentially. The more severe the environmental conditions, the greater the death rate.
Chemicals that are bactericidal substances can sterilize the surfaces of solids. High concentrations may be required so that the solution is not merely bacteriostatic. Chemicals more potent than disinfectants are typically required for sterilization since disinfectants kill vegetative cells but not necessarily the endospore of spore-forming pathogens. Chemicals can be expensive and present the problems of combining toxicity to microorganisms with safety to humans and the environment, instability, unpleasant fumes, proper disposal and undesirable staining or corrosive effects on dental and medical instruments.
Heat sterilization is a common method of sterilizing bacteriological media, hospital supplies, medical and dental equipments any many other substances. Either moist heat (hot water or steam) or dry heat can be employed, depending upon the nature of the substance to be sterilized.
When moist-heat sterilization is used, it must be borne in mind that some bacterial endospore are capable of surviving several hours at 212.degree. F. (100.degree. C.). Therefore, for moist-heat sterilization, an autoclave, pressure cooker, or retort, with steam under pressure, is required to achieve higher temperatures. For example, may bacteriological media are sterilized by autoclaving with steam at 250.degree. F. (121.degree. C.), under 15 lb. pressure, for 20 min or more, depending upon the volume of material being heated. Some spores are capable of surviving moist-heat sterilization equivalent to at least 7 min at 250.degree. F. (121.degree. C.).
Steam methods for sterilizing dental instruments generally require long sterilization times, on the order of 30 minutes to one hour. The relatively high cost and complexity of such devices requires a central sterilization area and does not permit the placement of units in individual patient rooms or operating rooms. Due to the long cycle time, it is necessary for a practitioner's office to maintain extensive and redundant inventories of costly instruments. Another disadvantage is the need for plumbing and the need typically to vent to an external atmosphere, especially if chemicals are introduced into the steam. Steam methods have the particular disadvantage of corroding and dulling metal instruments. Autoclaving is particularly destructive to dental handpieces (air-powered drills) since moisture corrodes the internal air vane and bearings. It has been experienced that even when the tool is relubricated for protection, the turbine can be destroyed within several months. Furthermore, the common practice of relubricating the bearings before and after autoclaving is thought to possibly impede sterilization of internal handpiece components.
Hot-air sterilizers without forced air flow may be used to sterilize heat-resistant materials, and are particularly suitable for instruments made of carbon steel. Dry heat sterilization conventionally requires heating at higher temperatures and for longer periods of time than does autoclaving. Temperature of 320.degree.-330.degree. F. (160.degree.-165.degree. C.) for sixty minutes to two hours is generally employed in hot-air sterilization. It is believed that dry heat kills microorganisms through denaturation of protein which may involve oxidative processes. Air flow using such methods may result in a non-uniform heat distribution and thus a non-uniform temperature profile within the sterilizing chamber. This decreases system reliability, particularly with large chamber sizes.
Hot air sterilizers using forced air flow are in part dependent on their flow patterns. Different air flow patterns may be used, but each has its own set of problems. Sterilizer manufacturers have proposed the use of cabinets which require the use of specially formed components to generate particular recirculating air flows, adding to the complexity and cost of the device. Archer and Cox, U.S. Pat. No. 4,975,245, issued Dec. 4, 1990, and U.S. Pat. No. 4,894,207, issued Jan. 16, 1990, disclose in related patents an apparatus and process for a recirculating high velocity hot air sterilizer. Cox et al., U.S. Pat. No. 4,923,681, issued May 8, 1990, and U.S. Pat. No. 4,824,644, issued Apr. 25, 1989, disclose, respectively, automatic microprocessor control means and an insulation jacket and housing for a sterilizer of the above-referenced patents. All four patents were commonly assigned to Archer Aire Industries, Inc., Dallas, Texas.
Archer and Cox disclose a process and device for sterilizing metal dental instruments wherein hot air, introduced as jets moving at 1500 to 3000 feet per minute into a chamber, heats the chamber to between 350.degree. and 400.degree. F. (177.degree.-204.degree. C.) and is recirculated between the chamber and a heater. Archer and Cox disclose creating a turbulent air flow within the chamber through the cooperation of a pair of corrugated deflector plates placed in a certain, spaced relationship to each other. Archer and Cox disclose that the recirculated air emanates into the chamber from rectangular slots in the lower corrugated plate as a uniform series of mutually spaced, nonturbulent hot air jets. It is further disclosed that the upper corrugated plate may be replaced with a flat plate, with the resulting loss in air mixing efficiency compensated for by the use of a larger fan motor. Additional, external ducts recirculate air from the sterilizing chamber back to the heater. The process and device disclosed depends on the cooperation and alignment of components which have complex geometries and which are in addition to the minimum number of functional parts required, namely base, chamber, blower and heater means.
Commercially available forced air sterilizers are relatively expensive, complex, bulky, and still require as much as six minutes to sterilize unwrapped instruments when operating at temperatures as high as 375.degree. F. (190.degree. C.). See, e.g., The Cox Rapid Heat Transfer Sterilizer model product literature, Cox Sterile Products, Inc., Dallas, Tex. This product literature discloses a forced hot air sterilization device with air flowing at a rate of 2,500 fpm and removing turbulence from the air by a patented "jet plate". These forced hot air sterilizers are expensive, retailing for in excess of $3,000.
It has not previously been known how to combine the features which are desired in a hot air sterilizer of the air flow type, especially for the dental practitioner, for example, overall small size, small footprint, power efficiency, small number of parts, simplicity of design, low manufactured cost, and efficient heat distribution. Costs, size, simplicity and efficiency, of course, are important.